Add generic error message when this.markup=markup.something() is incorrectly set.
Add generic error message when this.markup=markup.something() is incorrectly set.

Problem is hard to diagnose without some hint from the application.

cortesi/countershape,mhils/countershape,mhils/countershape,samtaufa/countershape,samtaufa/countershape,cortesi/countershape